Having previously thrown in the towel on trying different HD800 mods, Currawong's recently posted photo showing felt applied to the flat ring surrounding the driver, plus some foam over the bulging section, inspired me to get back to cutting and listening... and cutting and listening...
I've settled on this (for now) and I am really pleased with it (thus far) - nothing but self-adhesive felt cut to a ring shape:
The outside diameter of the ring of felt is 2-3/4 inches and the inside diameter is 2-1/8 inches.
I managed to find two different pieces of glassware having the right diameters to trace the circles on the felt backing before cutting with scissors - simple!
This takes most of the "sting" out of the treble very nicely - and doesn't do much more than that (hopefully). Time will tell.
I ended up not putting anything on the "bulge" or elsewhere.
Joy!
Mike